I would make it something unusual, rather than somthing with kewl powerz. An adventure I ran a few years back had the PCs questing to find "The Heart of the Tehchan". The Tehchan, they discovered, were a jungle tribe, and the "Heart" was a 12 year-old girl who was their spiritual emblem/advisor and had been kidnapped by a local tribe. The same quest also involved them finding the Holy Waters of Calatar (which were just waters from a sacred elven spring) and the Ice Blade of the Imryn (which was indeed a magical ice sword, but removing it from its paradisical valley in the arctic meant that the valley and all its inhabitants would succumb to the freezing surroundings).
I am also reminded of a Shadowrun (or was it Cyberpunk?) adventure where the PCs have to recover a rare sample of DNA. It turns out that the DNA in question is actually a horse - the last one left alive in the world. You could easily set something similar up in a fantasy game.
Or you could also take the abstract route, and have the PCs hunt for something like "happiness" or "fulfillment". How many adventurers ever truly experience such things in their turbulent lives?